The invention relates generally to process monitoring during laser treatment of a metallic surface, and in particular to real-time monitoring of a laser shock peening process by analyzing laser plasma emissions.
Laser shock peening (LSP), also referred to as laser shock processing, is an effective way of improving fatigue life of a metal work piece. Presently LSP finds wide application in the aerospace and automotive industries as a method for improving the fatigue properties of various metallic components, such as aluminum alloys, steel alloys, titanium based alloys, and nickel based alloys, among others.
Generally, in LSP, a surface of the work piece is covered by an opaque layer and a transparent overlay. The opaque layer may include a black plastic tape or a black paint coated on the surface of the work piece. The transparent overlay generally comprises a layer of water disposed adjacent to the opaque layer. During the process, a high-power pulsed laser beam is focused onto the surface of the work piece. The laser pulse passes through the transparent overlay and is absorbed by the opaque layer, causing a rapid ablation of the opaque layer producing a plasma. The blow-off of the plasma from the surface of the work piece generates a high-amplitude pressure shock wave. The pressure shock wave travels in two directions: First, a compressive wave travels through the opaque layer into the work piece. Second, a shock wave is reflected from the tape and travels backward through the transparent layer. Due to shock impedance mismatch, this backward traveling wave is reflected by the transparent layer toward the work piece. The shock waves ultimately combine to impart plastic strain to the work piece. This results in the deformation of the work piece and imparts compressive residual stresses, which remain following processing. It is these compressive residual stresses in the work piece, which effectively reduce crack propagation rates in the work piece and, thus, improves ftaigue properties of the work piece.
If the pressure produced by the laser is insufficient, the desired changes in mechanical properties of the work piece will not be achieved. Therefore, it is desirable to have the capability of monitoring the pressure and shock wave strength during the LSP process. One approach known in the art involves using a quartz gauge for pressure measurements during laser shock processing. A quartz gauge is based on the piezoelectric behavior of quartz crystals. In this technique, a quartz crystal is disposed on one surface of the work piece to be processed. When a pressure shock wave is applied to a surface of the quartz crystal by a laser pulse, an electric current proportional to the stress difference between the affected surface and the opposite surface is produced by the quartz crystal. The current flows through a resistor and the voltage measured across the resistor is proportional to the pressure response. By analyzing the pressure response of the quartz crystal, it is possible to determine shock-wave pressure produced on the work piece during the actual process. However this approach is disadvantageous because it is indirect and is performed offline, i.e. not in real-time. Moreover, such an approach is expensive, as the quartz crystal needs to be replaced after every laser shot.
Another technique to determine the quality of an LSP process includes performing accelerated fatigue test on a work piece after the work piece has been processed. However, since the LSP process and the work piece material are expensive, it is possible to sample only a limited number of parts for an accelerated fatigue test.
There is, hence, a need for a system and method for monitoring a laser shock peening process, which is inexpensive and is operable substantially in real-time.